1. Field of the Invention
The present invention relates to a sputtering film forming method, a sputtering film forming equipment, and a semiconductor device manufacturing method and, more particularly, an RF (radio frequency) sputtering film forming method, an RF sputtering film forming equipment, and a semiconductor device manufacturing method using the RF sputtering film forming method.
2. Description of the Prior Art
In recent years, semiconductor devices employing new material have been developed. For example, FeRAM (Ferroelectric Random Access Memory) in which information are stored by using its hysteresis characteristic has been known.
In the FeRAM, oxide compound material such as PZT, PLZT, etc. has been known as a dielectric film of a capacitor as a memory device. Also, advanced material which is formed by doping calcium (Ca) or strontium (Sr) into PZT has been known.
PZT is an abbreviation for "lead zirconate titanate (Pb(Zr.sub.x Ti.sub.1-x)O.sub.3)". PLZT is lanthanum-added PZT and its chemical expression is Pb.sub.y (Zr.sub.x Ti.sub.1-x)La.sub.z O.sub.3.
The sol-gel method and the sputtering method are known to form PZT, PLZT. Both methods have merits and demerits.
In the case of the sol-gel method, it has been difficult to improve device characteristics because of the problem of dust generation and organic component contained in the film.
In the case of the conventional sputtering technique, even though a target can be fabricated uniformly in conformity with composition, sometimes its composition is changed in long use. Also, it has been difficult for the sputtering technique to achieve desired film composition by controlling target composition.
As an approach for controlling an amount of Pb in the PZT film and the PLZT film, a method of growing the PZT film by controlling a substrate potential has been set forth in Patent Application Publication (KOKAI) Hei 5-78835 and Patent Application Publication (KOKAI) Hei 7-41944. In addition, a method of including excessive Pb or Pb oxide at initial growth of the PZT film has been set forth in Patent Application Publication (KOKAI) Hei 5-331638.
Further, a method of controlling an amount of Pb by changing the substrate potential has been described in FIG. 11 of Ohyo Butsuri, vol.65, No.12 (1996), pp.1248-1252.
The above dielectric film materials are mainly composed of oxide material. In case the dielectric film made of oxide is grown by the normal sputtering method, an insulating oxide compound target is employed. The oxide compound target is an electrically insulating target, so that it is impossible to execute the sputtering by applying DC power to the oxide compound target.
Therefore, if the sputtering is carried out by using the insulating oxide compound target, normally the RF sputtering method has been employed.
However, in the RF sputtering method, there has been the problem that discharge state is ready to change in time. This is due to the fact that a discharge potential is changed gradually because a film is grown on a chamber wall of the sputtering equipment.
Besides, if the plasma generated by applying the RF power is changed in time, a plasma potential or an electron temperature in the plasma is also changed to thus cause a change of a self-bias on the wafer. Then, if the self-bias is changed, it becomes very difficult to control composition of the oxide compound formed on the wafer. Especially, in case the PZT film and the PLZT film are to be formed, when an ion energy is made higher on the wafer, Pb is etched selectively on the wafer and an amount of Pb in the film is reduced smaller than a desired amount since Pb has a higher sputtering yield than Zr or Ti.
Therefore, according to the change of the self-bias, an amount of Pb in the PZT film and the PLZT film is changed gradually with the progress of film forming time. If such amount of Pb is changed, variation is caused in electric characteristics of the FeRAM to raise the problem.
For example, the method of growing the PZT film by controlling the substrate potential has been set forth in above Patent Application Publication (KOKAI) Hei 5-78835 an d Patent Application Publication (KOKAI) Hei 7-41944, but it is not expected that an amount of Pb should be controlled freely. In addition, if an electrostatic chuck is attached to the substrate side to improve control of the substrate temperature, film quality control mentioned in the above Publications is made impossible since its resistance value is high such as 1.times.10.sup.9 .OMEGA.cm to 1.times.10.sup.12 .OMEGA.cm. Furthermore, since the plasma in the chamber is gradually changed in case the insulating film is adhered onto the chamber wall, it is difficult to suppress the variation of Pb if such variation is checked over a long period of time.
In contrast, according to the method of controlling an amount of Pb by changing the substrate potential set forth in FIG. 11 of J. Appl. Phys., vol.65, No.12 (1996), pp.1248-1252, such an event has been described particularly that an amount of Pb can be increased up to 0.5 to 1.1 by inserting a variable resistor between the substrate susceptor and ground.
In this method, the RF current flowing into the substrate is controlled. Such control can be carried out in a device structure in which the RF current is easy to flow into the substrate, but the RF current flowing in the substrate side is reduced to make Pb control difficult if the structure by which the RF current can flow more easily to the chamber wall, etc. exists. In addition, if the electrostatic chuck is attached to the substrate side to improve temperature controllability, a resistance in this portion becomes an issue and Pb control becomes impossible. In other words, if potential of the chamber wall is a floating potential and also the substrate is directly connected to ground via the variable resistor, such approach becomes effective. However, for example, in case the chamber wall is at ground potential and the electrostatic chuck having a high resistance is attached to the substrate, an amount of Pb cannot be controlled, as described above. Moreover, when the film is deposited on the chamber wall, variation of Pb cannot be suppressed over a long period of time.
In the meanwhile, normally the plasma must be confined in the inside of a shield to surround the chamber. However, if the plasma in the chamber becomes unstable, the plasma leaks out of the shield to bring about adhesion of the film on the chamber wall and to cause sudden particle generation. Further, if leakage of the plasma out of the shield is not uniform, variation of film formation distribution on the wafer is generated. Also, in the sputtering equipment using a magnetron, since normally the film is formed by rotating the magnet, deviation of the plasma is ready to occur, which has an influence on the film formation distribution.
Leakage of the plasma out of the shield depends upon size and structure of the shield. The probability to leak out of the shield becomes higher as the pressure is increased higher, whereas the probability to leak out of the shield becomes lower as the pressure is reduced lower.
Accordingly, if process parameters are changed to control an appropriate amount of Pb, because the plasma is easy to flow out to the outside in a high pressure range, distribution of film thickness and film quality tend to become uneven to thus narrow the process window.
In particular, as disclosed in Patent Application Publication (KOKAI) Hei 5-331638, in case excessive Pb or Pb oxide is grown in an initial layer in sputtering film formation, the narrow process window makes it impossible to control an amount of Pb having good distribution by changing the sputtering conditions. This is because, if the film forming conditions are changed, a mode of the plasma is changed and the plasma is easily apt to deviate. In case the sputtering distribution is not good, it becomes impossible to grow only the initial layer in the Pb-excessive state.
As the countermeasure of this, the method of controlling an amount of Pb by changing the substrate potential has been set forth in FIG. 11 of J. Appl. Phys., vol.65, No.12 (1996), pp.1248-1252. But this method has disadvantages described as above.